The Local Area Network (LAN) is the heart of the office automation movement. There are many different approaches being marketed and all the ones with acceptable performance attributes for a LAN require installation of the transmission medium (whether it be coaxial cable, twisted pair or fiber optic cables). In a generalized application environment (regardless of the transmission medium selected) the installation of the transmission medium is both costly and inconvenient. Thus the concept of using in-place wiring to construct a LAN has much appeal. This basically defines two possibilities, (a) electrical power wiring and, (b) telephone wiring. Due to a number of practical problems, electrical power wiring is quickly eliminated as not interesting for a generalized LAN environment. Telephone wiring, on the other hand, is an excellent candidate. It is an ubiquitous resource that exists at any conceivable location for a workstation. It is also a highly under-utilized resource, both in terms of wires that are not used and, for those that are used, in terms of the bandwidth that is utilized.
For many years, the standard way of using telephone wiring for data communications was, and continues to be, via carrier modulation-demodulation techniques using modems. More recent techniques use short haul modems with carriers well above the voice frequency range and allow simultaneous voice and data communications on the same lines. The main problems with these techniques for LAN type applications are that they: (a) have very limited bandwidth, (b) require relatively expensive electronics, and (c) are fundamentally point-to-point methods as opposed to possessing the general interconnectability attribute desired for LANs. The disclosed technique, on the other hand, will enable a completely general LAN architecture with any access protocol and, in particular, is well suited to CSMA/CD (carrier sense, multiple access with collision detection). As a digital baseband system, it provides much greater bandwidth and, at the same time, costs much less than modem techniques. Because it is a baseband system, the allowable length of the communication links for the disclosed system will be more restricted than with modulated carriers, but will be quite adequate for virtually any foreseeable installation. For example, at 300 KHz, there should be no electrical problem to using links whose length is well over 1000 feet (305 meters). Relative to allowable error rates, the intelligence of the distributed resources in LAN applications allows for very error resistent communication protocols to be utilized. Consequently, extremely low error rates are not required.